As is known, in the field of microelectronics, the increase in the operating frequency of electronic systems has led to signal flows being provided in optical form by means of optical waveguides, which also have the advantage of providing high levels of immunity to electromagnetic interference.
This has led to the appearance of optoelectronic microsystems that can combine optical components, optoelectronic components (with either optical or electrical input or output signals) and electronic components (integrated circuits).
The overall optical alignment of the components is a critical stage in the assembly of this type of optoelectronic microsystem, and one of the major problems is that such overall alignment depends on a succession of two-by-two alignments between the successive optical and opto-electronics components of such a system. The optical alignment criterion for a system therefore depends both on the accuracy required for each two-by-two alignment and the number of components to be optically aligned.
Currently, microsystem alignment techniques can be divided into three major categories:
1—active alignment, for which light must be injected at the end of one fibre and the components positioned one-by-one in such a way that there is as strong a signal as possible at the end of the other fibre; optimization is carried out by small movements of the components to be aligned transverse to the optical path, which requires mechanical or piezoelectric micromanipulators. The positions defined in this way are then fixed by bonding. This type of active alignment procedure takes a very long time and requires mechanical fixing means the use of which does not generate stresses that might alter the final alignment (and which, of course, do not damage the components and in particular the fibres).
2—optical alignment using aiming sights which require highly sophisticated positioning equipment.
3—alignment using positioning blocks or stops made in the microsystem support; however, it is difficult to make blocks or stops on optical components.
A new method of passive alignment is beginning to emerge and is in particular described in document FR-2 757 276 (EP-0 944 851 or U.S. Pat. No. 6,151,173) relating to an assembly of optically aligned optical components and a method of producing it. To align two components mounted on the same substrate that must be aligned parallel to the surface of the substrate, the document teaches the use of micro-pellets made of a meltable material connecting positioning studs placed respectively on the surface of the substrate and each of the components. When the material melts between two facing studs, the surface tension forces of the material and the wettability of the material on the studs bring about the self-alignment of the components on the substrate holding them (the substrate is known as the interconnection substrate). Sub-micron accuracy is obtained.
This possibility of passive alignment has also been applied in Holm, Ahlfeldt, Svensson and Vieider's document “Through-etched silicon carriers for passive alignment of optical fibers to surface-active optoelectronic components” published in Sensors and Actuators 82 (2000) pp. 245-248, and in Souriau, Cobbe, Delatouche and Massit's “Passive Fibre Alignment on Optoelectronic Components for Electro-Optical Links Based on Single-Chip Technology and VCSELs” paper given at the 2001 Strasbourg IMAPS Conference. Here, the properties of self-alignment using micro-pellets are used to align an optical fibre and an optoelectronic component. In the first document, the fibre is engaged in a hole made in the substrate on which the component is self-aligned, whilst in the second document the fibre is engaged in a plate that is mounted on the substrate independently of the component.
What the above examples of passive alignment have in common is that they only propose the alignment of two components with each other, and the two aforementioned articles, which are the only ones to envisage mounting components perpendicular to a substrate, are limited to the alignment of two components (fibre+component) as they are limited to optoelectronic components solely carrying out an optical/electronic conversion or vice versa.
However, a new need has emerged to couple three optical components with very good optical alignment, namely two optical fibres (more generally two optical inputs/outputs) between which at least one at least partly optical component is interposed, within a small and reasonably priced optical arrangement which is easy to produce.